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Unless we are talking about MAC and
cheese, or the Mickey D kind of MAC, the aircraft MAC is the Mean Aerodynamic
Chord. This MAC is the width of the wing when measured through the center of
the wing in the forward-aft direction. On a plane like a Piper, this is just
the width of the wing. With a more complicated wing design like the Lancair it
is the average of this measurement. That is where the word “Mean”
comes from. This measurement has nothing to do with the “neutral point”.
It really just describes how effectively wide the wing is. The CG (Center of
Gravity) is the point around which the airplane balances (or would balance) if
it is sitting on its wheels. (Maybe that is a “neutral point”?) This
CG is calculated when the plane is motionless on the ground and on scales. It is
not the CG that the plane is operating with when it is in flight because the
horizontal stabilizer is usually designed to place a down force on the plane,
which will have the effect of moving the CG backward in cruise. That is why
the CG is specified to be in the front 25% of the wing width (MAC) in the
specs.
When we determine thru the Weight and
Balance calculations, the CG, we have no idea what the CG of the plane will be
in flight because as the angle of attack moves the Center of Lift forward and
aft, and the horizontal stabilizer adds and removes loads, we have no way of
calculating or knowing how these forces are moving. Hopefully the aircraft
designer did all this when he specified the CG range that we should keep the
plane in when it is on the ground and on its wheels. I suggest we stay inside
these recommendations.
Bill B
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Chris Zavatson
Sent: Thursday, July 15, 2010
11:30 AM
To: lml@lancaironline.net
Subject: [LML] Re: Small tail, MK
II tail, CG range
The aircraft MAC (also called neutral point) relative to CG
is the key to evaluating aircraft longitudinal stability. This is
independent of whether the tail is providing an up or down force
(either can be stable). Longitudinal stability is defined by the
reaction of the entire airframe to a disturbance from equilibrium. The
size, location and pitching moment characteristics of each component
factors in (wing, tail, fuselage etc.). Evaluating the behavior
of just the wing is not sufficient to describe the response of the
aircraft as a whole and certainly not to quantify the response. Actually,
a wing section alone will be unstable as the pitching moment is
negative. It is stable when inverted - flying wings have negative camber
for this reason.
A stable aircraft must have a positive
pitching moment when in equilibrium. In order to be stable, the
pitching moment coefficient must have a negative slope with
increasing angle of attack. This provides an increasing
opposing moment to an increasing disturbance.
A larger tail increases the response when a disturbance
occurs. It is a function of the larger area producing
more restoring force for any given angular
disturbance. The size of the horizontal stabilizer feeds
into a quantity called the tail volume ratio - a unit-less measure of
relating tail area to wing area and wing mean wing chord to distance to
the horizontal stabilizer. More area or a longer tail increase the
effectiveness in terms of stability.
The neutral point is fixed by the configuration of
the aircraft. Only configuration changes will move the neutral
point. Lowering the flaps, for example, changes the airfoil, relative
incidence angles, pitching moment of the wing and so on. In all configurations
the neutral point must remain well behind the CG. 10% of the mean chord
length is a good starting minimum. Once the neutral point is known, the
incidence angles and CG can be set. What will fall out is the trim
airspeed. That is, in equilibrium the aircraft will seek out a
specific angle of attack and the corresponding airspeed. One can play
around with different combinations of incidence angles and CG
locations to achieve both a stable aircraft and minimum trim drag at any desired
airspeed.
From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Wed, July 14, 2010 10:37:18
AM
Subject: [LML] Re: Small tail, MK
II tail, CG range
I'm not familiar with MAC as applied to the entire airframe,
can you elaborate? I think there may be a problem with that idea since the tail
is typically providing a down force which would move the "airframe MAC" to
the front, not the rear.
----- Original Message -----
Sent: Tuesday, July 13,
2010 8:35 PM
Subject: Re: [LML] Small
tail, MK II tail, CG range
<<Any more to the rear and you get negative stability
at cruise and a larger tail doesn't help much with that anyway.>>
A larger tail moves the MAC rearward allowing the CG to
move farther aft while maintaining the same level of stability.
There has been a lot of discussion about Cm. We
need to be careful to distinguish between the Cm for the wing, tail and total
aircraft. It is the later that is critical to stability and this is where
the larger tail influences the situation. The large tail moves the MAC to
the rear approx. 1.5 inches. For the same CG, the more rearward MAC
produces a greater restoring force if the plane is disturbed from level
flight. The practical benefit for us is that it allows a lot more
baggage to be thrown the rear of the plane before suffering stability
problems. You pointed out the other benefit of increased control
authority at slow speed with full flaps.
From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Tue, July 13, 2010 2:51:23
AM
Subject: [LML] Small tail, MK II
tail, CG range
I'm checking further into the data on these questions and am coming to
question the need for a larger tail. I'm not sure a larger tail by itself will
solve the problem. After doing some static and in flight measurements, it looks
like the tail authority is not a big problem, if a problem at all.
Static measurements of N31161 have shown "vanilla"
parameters. 2.5º incidence between the wing root at full reflex and the tail
and a 1.3º washout. Put the flaps at 0º and you get an additional AoA of 1.8º
at the root for a total incidence of 4.3º . . . . not radical at all.
What is interesting is the POH (Dec. 1994 pg. VI-3) gives the CG range
as 24.5" to 30.3" aft of the rear face of the fire wall and the MAC
at 15% to 20%
. . . well . . . no . . . that range is more like a MAC range of 15% to
30% - - - a good range made touchy only by the small size of the air frame.
After going over the plan view kit drawings, I come up with a CG range
of 23-1/4" to 29-1/4" for a MAC range of 15% to 30%
That range is about 1-1/4" forward of the book and fits better
with first hand flight experience.
Any more to the rear and you get negative stability at cruise and a
larger tail doesn't help much with that anyway.
Negative stability makes pitch control a real chore. As Scott K. has
indicated, going to 0º flaps helps under that loading condition.
Too far forward and landing becomes "interesting". A larger
tail can help there . . . or don't use as much flaps.
I think understanding these conditions can help everyone.
. . . The quest continues . . . Comments welcome.
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From:
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"Wolfgang" <Wolfgang@MiCom.net>
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Sender:
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<marv@lancaironline.net>
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Subject:
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Small tail, MK II tail, CG range
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Date:
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Sat, 10 Jul 2010 21:01:11 -0400
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To:
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lml@lancaironline.net
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The LNC2 uses the NLF(1)-0215F airfoil. A lot can be
found by doing a Google search on that number.
More detail can be found by going to Google for
"NASA Technical Paper 1865".
I have not taken the time to reverse engineer the CG
range of the LNC2 but let me offer some observations.
The airfoil used has long been touted as "the
greatest thing since sliced bread" for General Aviation and it
definitely has some advantages. But it's not new. Compare this airfoil to the
P-51 airfoil and you will see some close similarities. The LNC2 being
composite construction instead of aluminum lets the airfoil show more of it's
theoretical advantages.
It's a laminar shape with a good drag bucket. That
bucket can be made to move to the lower Cl (lift coefficient) ranges with
reflex allowing noticeably lower drag at higher cruise speeds. Along with
reflex, the Cm (moment coefficient) goes positive, the center of lift of the
wing travels forward giving a nose up force requiring down trim. This is in
addition to the usual nose up force that goes with most all airfoils at
high speed before considering flaps.
With down flap, the drag bucket will move to higher
Cl's making slower flight more efficient. And, of course, the Cm goes
negative giving a nose down force requiring up trim.
. . . and appropriate variations in-between . . .
So, the rear CG limit is determined by high speed
flight and available control authority,
and the forward CG is determined by low speed /
landing flight and available control authority.
What is becoming clear here is that the center of
lift does quite a bit of traveling fore and aft which is exaggerated by
allowing negative or "cruise" flaps. Since you can't shift the CG
during flight, you need a large amount of pitch authority from the tail to cover
that range of lift travel.
You have two choices in the LNC2, live with the
limitations or install a larger tail to give that extra pitch authority.
. . . A larger tail area can also help
with abnormal attitude recovery.
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